Package crypto implements a simple convenience wrapper around golang.org/x/crypto/nacl/box. It ultimately models a situation where you don't care about authenticating the encryptor, so the nonce and encryption public key are prepended to the encrypted message.
Shared key precomputation is used when encrypting but not when decrypting. This is not an inherent limitation, but it would complicate the implementation a little bit to do precomputation during decryption also. If performance becomes an issue (highly unlikely), it's completely feasible to add.